This invention relates generally to wave filters, and particularly to radio frequency filters of the kind comprising two or more dielectric resonators. The radio frequency filters according to the invention lend themselves to use in mobile or portable telephone sets, among other applications.
Bandpass or bandstop radio frequency filters have been known which are each comprised of a juxtaposition of coaxial dielectric resonators operating in transverse electromagnetic (TEM) mode. Examples of such filters are disclosed in the article entitled "Radio Frequency Circuit Components" by Nishikawa in Microwave Workshop Digest, MWE '91. The coaxial dielectric resonators in such filters are coupled together via capacitors, strip transmission lines, transformers, or the like.
The current trend with such dielectric resonator wave filters, as with almost any other electric or electronic devices and appliances, is reduction in size. This trend inherently requires the juxtaposition of the coaxial dielectric resonators as close as possible. Conventionally, however, the closer the resonators were juxtaposed, the less were their terminals electrically isolated from one another. The result was the danger of the leakage of the frequencies that had to be attenuated, from the input side to the output side terminals. This phenomenon is due obviously to the aerial propagation of signals.
It might then be contemplated to provide the resonators within antileakage shields of one kind or another. This solution works to a certain extent, but too much reliance on such shields is objectionable because they not only make the complete apparatus heavy, bulky and costly but also set limits on the latitude of filter design in meeting various requirements of each specific application.
The same problem existed with filter systems each comprising a required number of dielectric resonators mounted on a common base structure to make up two or more filter units. Signal leakage was easy to occur in this case from one filter unit to another wherever the resonators were closely juxtaposed.